1. Field of the Invention
The present invention relates to a method of continuously preparing a film made of a thermotropic polymer capable of forming an optically anisotropic melt phase.
In the following description of the present invention, the thermotropic polymer capable of forming an optically anisotropic melt phase is sometimes referred to as a “thermotropic liquid crystal polymer” unless otherwise specified and, also, the film made of such thermotropic liquid crystal polymer will be referred to as a “thermotropic liquid crystal polymer film” unless otherwise specified.
2. Description of the Related Art
The thermotropic liquid crystal polymer film is known to have an excellent low moisture absorbability, a high beat resistance, a high chemical resistance and excellent electrical properties and is therefore rapidly commercialized as an electric insulating material that improves the reliability of printed wiring boards or the like. In recent years, in the field of the electric and electronic appliances, the SMT (surface mounting technology) has come to be widely utilized and is currently largely applied in the production of electric and electronic appliances. As a result, the packaging density of electric and/or electronic component parts on a circuit board has drastically increased and lighter, more compact high-tech products hitherto considered unable to make have come to be developed. Accordingly, the need has arisen to improve the accuracy of dimension of the circuit boards.
When it comes to mounting electric/electronic component parts such as, for example, semiconductor elements and resistance elements directly on a surface of a circuit board, the presence of a difference in coefficient of thermal expansion between the circuit board and the electric/electronic component parts tends to bring about a problem associated with displacement in position of those component parts relative to the electric printed circuit board.
In order to resolve such problem, attempts have made to heat treat a laminated circuit board, made up of a thermotropic liquid crystal polymer film and a metallic foil, prior to electric/electronic component parts being mounted on the surface thereof, so as to render the film, forming a part of the laminated circuit board, to have a thermal expansion coefficient to be substantially equal to that of the electric/electronic component parts, such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 10-157010. Also, U.S. Pat. No. 5,529,740, issued Jun. 25, 1996, to Jester, et al., for example, discloses the heat treatment of a laminated product, including the thermotropic liquid crystal polymer film and a support body made up of the metal foil, to thereby improve the physical properties of the thermotropic liquid crystal polymer film.
According to the Japanese laid-open patent publication referred to above, it is described that if the thermotropic liquid crystal polymer film forming apart of the laminated circuit board has a thermal expansion coefficient larger than that of a component part to be surface mounted, the heat treatment of the laminated circuit board is carried out at a temperature within the range of the temperature, which is lower by 140° C. than the melting point of the film, to the melting point thereof and that by so doing, the thermal expansion coefficient of the film could be lowered by 18×10−6 cm/cm/° C. at the maximum. It is also described that if the film has a thermal expansion coefficient smaller than that of a component part to be surface mounted, the heat treatment of the laminated circuit board is carried out at a temperature within the range of, the melting point of the film to the temperature, which is higher by 20° C. than the melting point thereof, to thereby render the film to have an increased thermal expansion coefficient.
On the other hand, the USP referred to hereinabove discloses heating of a thermotropic liquid crystal polymer film, held in contact with a support body, at a temperature higher than the melting point of the film to thereby melt the latter and subsequent cooling of the melted film to form of a solidified polymer layer. This USP also discloses various physical properties of the resultant film and describes that the thermal expansion coefficient of the film could have been increased by means of the heating and cooling carried out in the specified manner.
However, as is well known to those skilled in the art, the thermotropic liquid crystal polymer, when formed a film by the use of an extrusion molding technique such as, for example, an inflation molding, generally results in the film having a negative coefficient of thermal expansion. When it comes to the use of such thermotropic liquid crystal polymer film as an electric insulating layer on the circuit board, in order for the thermotropic liquid crystal polymer film to have a thermal expansion coefficient matching with that of component parts to be eventually surface mounted on the printed circuit board, the thermal expansion coefficient of the thermotropic liquid crystal polymer film must be increased during the course of manufacture of the circuit board. Although as hereinabove discussed, the thermal expansion coefficient of the thermotropic liquid crystal polymer film can be adjusted by means of the specific heat treatment, it is generally understood that to increase the thermal expansion coefficient of the thermotropic liquid crystal polymer film the heat treatment must be carried out at a temperature equal to or higher than the melting point thereof as disclosed in any one of the previously discussed patent literatures.
It has, however, been found that the heat treatment of the thermotropic liquid crystal polymer film at such a high temperature involves numerous problems associated with the manufacture thereof on an industrial scale. Specifically, as a result of excessive thermal load imposed on the film, the resin tends to be quickly deteriorated; the higher the temperature of heat treatment, the more difficult to control the thermal expansion coefficient of the film precisely; inconveniences such as, for example, film deflection tend to occur easily; a substantial amount of energies is required to accomplish the intended heat treatment; and apparatuses used to manufacture the thermotropic liquid crystal polymer films are limited to result in increase of costs.